Most investigators agree that pigments, in particular certain carotinoids, must be involved in the entrainment of physiological responses, but the pigments themselves and the means by which light regulates enzymatic processes is unknown. Our previous studies have used, as the parameter of measurement, the circadian output of serotonin. We have demonstrated that the cycle of serotonin output in the organ culture is regulated by the photoperiod. Our preliminary studies with organ cultures have demonstrated that the wavelengths of light play an important role in the amount and pattern of serotonin output. In this proposed investigation the 24-hour serotonin rhythmic patterns of production will be studied in seven monochromatic light conditions. By studying the results of the monochromatic light in the 24-hour production of serotonin, the type and character of the pigmental system that is interpreting photoperiod information for the organism will be investigated. The conversion of light energy into high energy molecules must require pigmental cofactors such as the pterins. The influence of the pterin, folic acid, will be studied in the proposed investigation. By adding folic acid at time of low and high periods of serotonin production and measuring any change in the levels of serotonin output, folic acid will be assumed to be involved in the intracellular regulation of serotonin. The influence at the enzyme levels of folic acid will be determined by the use of timed studies of breakdown of labeled tryptophan and the increase in labeled serotonin. The isotopic materials will be used in organ cultures and the production of the labeled products will be measured in tissues that are known to produce serotonin in rhythmic patterns in 24 hours. The results of this study will aid in the understanding of the regulation mechanisms of rhythmic serotonin production. The research should be of value to investigators of neuroamine physiology as it relates to mental health and behavior, neurophysiology and pharmacology and physiological rhythmic phenomena.